U.S. Secretary of Defense Ash Carter speaks with U.S. service members aboard the USS Theodore Roosevelt aircraft carrier in the South China Sea, in this handout photograph taken and released on November 5, 2015.

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U.S. Defense Secretary Ash Carter removed his senior military assistant, Lieutenant General Ron Lewis, from his position on Thursday over allegations of misconduct that Carter learned of this week.

Carter has referred the matter to the inspector general of the Defense Department, he said in a brief statement on Thursday. After the inspector general's investigation is complete, the Army may take action, a senior defense official said.

As senior military assistant, Lewis served as a top adviser to Carter on military matters. He accompanied Carter last week on his trip to Asia.

Carter referred to "allegations of misconduct" but he did not specify what the allegations referred to. A senior defense official also declined to specify the nature of the allegations.

"I expect the highest possible standards of conduct from the men and women in this department, particularly from those serving in the most senior positions," Carter said in his statement. "There is no exception."

The senior defense official said Lewis would report to the Army Vice Chief of Staff while the allegations are investigated.

"The secretary was very surprised to learn of these allegations," the official said.

The inspector general's office confirmed it had received a referral from Carter's office regarding Lewis and said it would conduct an investigation. It gave no further details.

Carter first learned of the allegations on Tuesday evening and spoke to Lewis on Thursday morning, the senior defense official said.

Lewis, an Army air cavalry officer, has served as Carter's trusted aide on and off for years, including as his military assistant when Carter was undersecretary of defense for acquisition, technology, and logistics.

Lewis deployed to Afghanistan as deputy commanding general of the 101st Airborne Division, returning in February 2014.